Many types of electrostatic filters have been proposed for removing small particulate materials such as dust, smoke, and the like from gases such as air or the exhaust gases of vehicles or industrial processes. Typically, such filters rely in one way or another on the ionization of the particulate material by a fixed high voltage electric field, so that they may be trapped and held by electrostatic forces. Common disadvantages of ionizing electrostatic filters are that they operate at sufficiently high voltages, requiring expensive insulation and safety precautions, as well as substantial power, and that they produce ozone, which constitutes a health hazard. There are a number of additional problems with known electrostatic filter technologies, whereby the attraction and collection of particulates to the filter materials are accomplished by Coulomb's Law, including flocculating effects, creating unpredictable occasional bursts of release of dust, inadequate dust-holding capacity, requiring more frequent maintenance, and other common disadvantages associated with high voltage utilization. Thus, electrostatic filtration is used today mainly as a pre-filter or general purpose filter for commercial purposes, without requiring realistic high performance.
Non-ionizing electrostatic filters have also been proposed in the past, but their use tends to be limited to special situations, such as the capture of partially conductive soot particles from diesel exhaust.
Mechanical filters (including high efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filters not using electric fields are also common, but they are basically unable to capture particles smaller than their pore size; and they are also subject to rather rapid clogging by captured particles. The clogging takes place mostly on the inflow surface of the filter, and the thickness of the filter material for holding particles is not utilized as it would simply increase the air pressure drop across the filter.